As an important type of steel, the high-strength low-alloy steel, is applied widely to fields like military industry, automobile industry, mining machinery, engineering machinery, agricultural machinery and railway transportation. With the rapid development of China industry, various military and civil equipments become more complicated, larger and lighter, which requires high-strength low-alloy steel plates used for making the equipments, not only to be of higher hardness and strength, but also good toughness and forming performance. In recent decades, the research and application of high-strength steel plate develops very fast. This type of steel is developed on basis of high-strength low-alloy weldable steel, and the service life thereof is many times longer than that of traditional structural steel plate; the manufacturing process thereof is simple, which normally includes cooling or quenching directly after rolling, or offline quenching and tempering, or controlled rolling and controlled cooling to strengthen.
In traditional process of manufacturing high-strength low-alloy steel for automobiles, engineering machinery, and warship hull structures, many expensive alloy elements such as Cu, Ni, Cr and Mo are added, which cost much. Currently, high-strength steel begins to develop in two directions, one of which is low-cost production, and another is high cost with high performance. In China, when producing high-strength steels, steel mills prefer to add alloy elements like V, Ti, Cr, Si, Mn, B, RE which are abundant in home, and the addition amount is normally ≦3%. As to those high-strength steels with higher strength in warship hull structures, automobiles, mining machinery, engineering machinery and the like—for instance, steel plates with yield strength of 700 MPa,—elements such as Cu, Ni, Cr, Mo and the like are further added to improve its property. Although the yield strength of the steel plate is up to 700 MPa, its low-temperature toughness is not high enough for military warship hull structures and civil equipments Which have strict requirements on low-temperature impact at −60° C. or even −80° C. Now, in China, high-strength steel with yield strength of above 700 MPa, are still dependent predominantly on imports.
HSLA−80/100 in United States Military Standard MILS-24645A-SH relates to a type of steel, in which C≦0.06%, Si≦0.04%, Mn: 0.75-1.05%, P≦0.020%, S≦0.006%, Cu: 1.45-1.75%, Ni: 3.35-3.65%, Cr: 0.45-0.75%, Mo: 0.55-0.65%, Nb: 0.02-0.06%, minimum Ceq is 0.67 and plate thickness is ≦102 mm, which adopts the alloying design of low carbon or even ultra-low carbon (C≦0.06%), to ensure the excellent weldability and low-temperature toughness. In the steel, high content of copper and nickel are added, wherein owing to the age hardening of copper, high strength can be obtained without obvious damage to its toughness and plasticity. It has a yield strength of 690-860 MPa, an elongation of 18%, an transverse Akv at −18°0 C. of 108J and an transverse Akv at −84° C. of 81J. Due to that a lot of expensive alloy elements are added therein, it becomes very costly.
Now, in patent documents relating to high-strength high-toughness steel plates with yield strength of about or above 700NIPa, which have been published, WO 200039352A, for example, discloses a low-temperature steel, wherein high-strength steel with tensile strength of above 930 MPa and good low-temperature toughness, is obtained through adding low content of carbon (0.03-0.12%) and high content of nickel (no less than 1.0%) and adopting a low cooling rate (10° C./s).
WO 9905335A discloses a high-strength steel with relatively low content of carbon (0.05-0.10%) and high content of Mn, Ni, Mo and Nb. After rolling, the steel is only quenched, but not tempered, such that the tensile strength thereof can be up to above 830 MPa, and the minimum Charpy impact energy at −40° C. is 175J.
Currently, it is still necessary to provide a medium steel plate with high strength and toughness which is relatively economical and can be applied widely in industries such as automobiles, engineering machinery and warship hull structures.